8o UNDERHILL : 



raentary or splint bones, which is the foot structure of our 

 modern horse, ass, and zebra. This evolution of the horse 

 family took place in America between the Early and Late 

 Tertiary, during the elevation of the continental land surface, 

 and was accompanied by a coordinating increase in size and 

 adaptive modification of the animal to its changing condi- 

 tions of life. 



Related to the horses and radiating from the same central 

 Phenacodus type we have two other living odd-toed (perisso- 

 dactyl) families, represented by the rhinoceros with three 

 fingers and three toes, and the tapir with four and three 

 respectively. During most of the Tertiary rhinoceroses were 

 common beasts in North America as well as in the Old World, 

 but before its close they had become extinct in America, 

 while in the Old World they have survived to the present 

 time. The tapirs began in the Early Tertiary and exist 

 to-day in South America and India ; they are the survival 

 of a very ancient ungulate type, related to the extinct Cory- 

 phodon and Palcsotheruim , the latter appearing to have agreed 

 with the modern tapirs in possessing a lengthened and flexible 

 nose, a sort of trunk, suitable for stripping foliage. Although 

 the tapir has four fingers, the axis of the arm passes through 

 the third, and the number of toes is three. Thus it exhibits 

 in the fore limbs the relation of parts, and in the hind limbs 

 the structure as well as the relation of parts which distin- 

 guishes the odd-toed from the even-toed branch of the Ungu- 

 lates. 



In the last named section the toes are two or four in 

 number (artiodactyl), and the axis of the limb instead of 

 passing through the third, as in the Perissodactyla, passes 

 between the third and fourth, which have a form in rumi- 

 nants which suggests the term "cloven-foot." We observe, 

 then, that in the first sub-order support is centred in the third 

 toe, which is the most highly developed, while in the second 

 the support is equally divided between the third and the 

 fourth, which have an equal development. The artiodactyl 



